The experience of photography outdoors typically has to contend with the sun. The bright, concentrated light from the sun, especially on a cloudless day, causes glare as it enters a camera lens, and washes out detail nearby to the sun in the camera image. In some cases, this means that a significant amount of the potential image is lost. The same phenomenon can be found with other bright light sources such as headlights and streetlamps in a night scene.
Bright lights cause many adverse effects in an electronic imaging system. In analog imaging systems such as vidicon tubes, the burned-in area from such a bright light source can causes a trail to be left behind when either the camera or the light source is in motion, and can even cause a permanently damaged spot on the tube. In charge-coupled-device (CCD) video systems, the sensor chips react to extreme light sources by what is called “spill” or “blooming” where the electrical charge caused by the light spills over into adjacent cells on the sensor, especially those directly underneath, leading to large vertical bright streaks in the image from the location of the bright light sources.
These bright lights also have adverse effects in lenses. They cause internal reflections among the glass lens elements which produce ghost images. In addition, the bright light interacts with the air-glass surface and produces a hazy spreading of the light, which is amplified by any dirt, grease or moisture on the lens.
Many modifications to an optical system have been attempted to cut down on glare. Polarizing filters cut down on stray light but cannot reduce the appearance of the sun sufficiently to make details in the image nearby it reappear. Lens hoods also cut down on stray light entering the scene from outside of the image field of view, but cannot compensate for any points of overexposure within the field of view itself. Neutral density filters and internal irises cut down on the light entering the lens, but at the expense of an overall darkening of the scene.
Other inventions have attempted to regulate the light in the image system. For example, Bechtal, et. al. in U.S. Pat. No. 6,863,405, uses a dimming mirror controlled by a glare sensor in a vehicle, to regulate the reflectivity of a rear-view mirror to prevent headlights from being reflected into the driver's eyes. This refers to an overall control applied to the mirror, based on an overall reading of brightness, especially when it is compared to the ambient overall lighting. When these are too disparate, they create what is called a glare threshold.
In camera systems, overall variation of the sensitivity of the sensor can be used to control the effect of bright lights shining into the lens. Auto-iris systems using a physical lens iris, or auto-exposure using faster or slower shutter speeds, both can regulate the appearance of a video image or other digital or analog image-making system. However, these apply only to the overall image, not to any portion of it in particular.
In another approach to regulating light within an camera, Karioja et. al. in “Optical Attenuator Using Liquid Crystal Film” describes using an a film of liquid crystal droplets, located between a lens an a sensor, as an overall variable light scattering device that would have the effect of regulating exposure. However, this too applies to the entire image, and does not affect the light entering the lens.